Read about range operators in perlop

.. will test both conditions on the first line that is examined. ... won't test the 'finish' condition on the first line.

Example:

begin a = b + 1 end
begin j = k - 1
end
[download]

/begin/ .. /end/ will identify the first 'end' as the finish condition, while /begin/ ... /end/ prevents checking for 'end' until a subsequent line is process. Hence, it finds the second occurance.

I also suggest you to read Flipin good, or a total flop? tutorial